1. Field of the Invention
The present invention relates to a constant velocity universal joint to be used as a component of a power transmission of a vehicle.
2. Related Art
Generally, the constant velocity universal joint to be arranged in the power transmission line of a vehicle is exemplified by the Birfield type, tri-port type, double-offset type, cross groove type and double Cardan type. Of these, especially the Birfield type constant velocity universal joint, as frequently employed on the wheel side of the front drive shaft of the vehicle, is equipped with an inner race and an outer race arranged on the outer side of the inner race. A plurality of inner grooves are formed in the outer circumference of the inner race, and a plurality of outer grooves are formed in the inner circumference of the outer race. Moreover, one inner groove and one outer groove makes a pair for retaining each ball.
In addition, a bisector is set at a position halving the angle which is made between the first axis of the inner race and the second axis of the outer race. Moreover, the center of curvature of the inner groove in a plane containing the first axis and the center of curvature of the outer groove in a plane containing the second axis are offset on the two sides of the bisector. Between the inner race and the outer race, still moreover, there is arranged an annular retainer. This retainer retains the individual balls.
With the drive shaft being mounted on the vehicle, moreover, the inner race is connected to the shaft on the differential side, and the outer race is connected to the wheel side. Here, the height of the connection portion of the shaft on the differential side and the height of the connection portion on the wheel side are made different. As a result, the first axis of the inner race and the second axis of the outer race intersect to set a joint angle.
According to the Birfield type constant velocity universal joint thus constructed, the torque, as outputted from the differential, is transmitted through the inner race, the balls and the outer race to the wheels so that the vehicle is run by the driving force of the wheels. During the transmission of the torque by the Birfield type constant velocity universal joint, each ball moves in a direction normal to the bisector while being retained by the retainer. Moreover, the center of each ball is retained in the bisector so that the constant speed rotations are kept between the inner race and the outer race.
In the Birfield type constant velocity universal joint, however, the curvature center of the plurality of inner grooves and the curvature center of the plurality of outer grooves are offset on the two sides of the bisector. As a result, at the torque transmitting time, two loads toward the center of the balls are established at the touch points between the inner and outer grooves and the balls.
Then, the ball is pushed by the resultant force of the two loads in a direction normal to the bisector so that the retainer is pushed onto the outer circumference of the inner race and the inner circumference of the outer race by that pushing force. As a result, the touch point between the retainer and the inner and outer races, the touch point between the retainer and the ball or the touch point between the ball and the inner and outer grooves cause frictions to generate heat and repeated loads to invite fatigue and separations. This causes various problems of the reduction in the durability or torque transmitting function of the Birfield type constant velocity universal joint or of an increase in vibrations or booming noises.
On the other hand, one example of the invention capable of suppressing the resultant force to act on the balls is disclosed in Japanese Patent Laid-Open No. 91458/1995 (JP-A-7-91458). Here is disclosed that an included angle that is, an angle, which is made between a segment from the touch point between the ball and the inner and outer grooves to the center of the balls and the bisector, is set at a small value. When this construction is adopted, the resultant force in the direction normal to the bisector is reduced to lighten the friction and the repeated load at the touch point between the retainer and the inner and outer races or at the touch point between the retainer and the ball.
In the disclosed invention, moreover, of the inner groove and the outer groove, the curvature center corresponding to the axial base end portion to abut against the ball entering the inside of the outer race is set at a position different from the curvature center in other portions. Specifically, the included angle corresponding to the base end portion is made larger than that corresponding to other portions. When the ball is brought into the inside of the outer race by the constant speed rotations of the inner race and the outer race, therefore, the ball lock is suppressed to suppress the heat generation more at the touch point.
According to the Birfield type constant velocity universal joint disclosed in the aforementioned Laid-Open, however, it is possible to suppress the resultant force in the direction normal to the bisector but not to reduce the load to act on the touch point between the ball and the inner and outer grooves. Thus, the load to occur at the touch point between each ball and the inner and outer groove has made it impossible to solve the problem that the heat generation, fatigue or separation is caused in the ball, the inner race or the outer race. It has also been impossible to solve the problem that vibrations and booming noises are raised during the torque transmission.
In the Birfield type constant velocity universal joint of the Laid-Open, moreover, the curvature center of the inner groove and the curvature center of the outer groove are offset on the two sides of the bisector. In other words, the touch point between the ball and the inner surface of the inner groove and the touch point between the ball and the inner surface of the outer groove are offset on one side of the bisector. As a result, at the torque transmitting time, two forces (or loads) act from the individual touch points to the center of the ball. Moreover, these two forces establish a resultant force in a direction normal to the bisector to push each ball.
Then, the pushing force of each ball is transmitted to the retainer so that the inner circumference of the retainer is pushed onto the outer circumference of the inner race whereas the outer circumference of the retainer is pushed onto the inner circumference of the outer race. This results in the heat generation at the touch portions between the retainer and the inner and outer races. As a result, fatigue wear or separations may occur at those touch portions to lower the durability and the torque transmitting function of the Birfield type constant velocity universal joint.